1. Field of the Invention
The present invention relates to an image reproducing apparatus, and in particular to a projection-type display apparatus for projecting an image to enlarge and display the same.
2. Description of Related Art
Heretofore a projection-type display apparatus for projecting an image of a small size and enlarging it optically, has been utilized for realizing a large image having more than 40 inches along a diagonal line. Small size liquid crystal panels are used often as light valves for forming a small size original image to be enlarged optically. That is, incident light from a light source is intensity-modulated by the liquid crystal panels. Intensity-modulated light thus obtained is projected on a screen, enlarging it optically by a projection lens, in order to realize a large image display.
Prior art techniques on such a projection-type display apparatus using liquid crystal panels are discussed in International Symposium Digest of SID, pp. 227-230 (1990). Hereinbelow this prior art techniques will be explained, referring to FIG. 25.
In FIG. 25, this apparatus includes 3 small size transmitting-type liquid crystal panels 1100, 1101 and 1102 with respective condenser lenses 1112, 1113 and 1114 driven in accordance with image signals corresponding to color images of red, blue and green, respectively, a light source 1103 of a metal halide lamp, a reflecting mirror 1104, 4 dichroic filters 1105, 1106, 1108 and 1109, mirrors 1107 and 1110, and a projection lens 1111. The optical system except for a screen 1120 is covered by a cover 1115.
On the other hand, as another prior .art technique, there is known a liquid crystal display apparatus disclosed in JP-A-1-195782. In this prior art technique a cathode ray tube was used as a light source instead of a discharge tube such as a metal halide lamp. In this prior art, no attention was paid to the life of the light source and the necessity of maintaining a sufficient amount of light and therefore it had a problem from a view point of downsizing and maintenance of the apparatus, as explained below.
At first, in this prior art, a light source such as a metal halide lamp, etc. is used. Therefore, although it is possible to generate light with a high brightness, since it emits white light, it is necessary to separate the white light into different color components by dichroic filters and to combine color components in order to display it. For this reason there were problems that the optical system is enlarged and complicated so that adjustment of the optical system is difficult and that mechanism and parts for adjusting the optical axis are required and thereby weight is increased. Also, the life of the metal halide lamp is as short as about 2000 hours. Consequently, supposing that it is used, e.g., 6 hours a day, the life of the lamp expires in about one year. For this reason it is inevitable to exchange it during the usable period of the optical system and an exchange frequency is as high as about once a year. Further, the metal halide lamp is expensive and in addition special techniques are required for handling and exchange thereof. Therefore, there was a problem that it is difficult for a user to handle or exchange it freely.
Further, the prior art apparatus had a problem that increase in the size and complication of the optical system including a position adjusting mechanism and the apparatus are inevitable, because it is necessary to always use different liquid crystal panels for the 3 primary colors, in order to obtain a bright color image.
In addition, in the liquid crystal display apparatus using a Braun tube, the problem to be solved is to compensate for slow response of the conventional liquid crystal display apparatus. Also, the Braun tube used there was that used in a usual television receiver and therefore there was a problem that it is difficult to obtain a brighter image, because sufficient light amount cannot be obtained when the Braun tube is used as a light source.
Further, in this prior art only one liquid crystal display panel is used. In the projection-type display apparatus, since a small-size liquid crystal display panel is used for the light modulation to reproduce a color image, the area of one pixel is extremely small. Therefore, decrease in the aperture ratio is caused, which reduces the efficiency of light utilization. Thus there was a problem that it is difficult to obtain a brighter image.
There is described a liquid crystal projection-type display apparatus using three TN-(Twisted Nematic-)type liquid crystal panels as an image source to obtain a color image by projecting it on a screen in detail in International Symposium Digest of SID, pp. 375-378 (1986).
Furthermore, recently, there is described a display apparatus using polymer-dispersed-type liquid crystal, in which liquid crystal is distributed in a transparent resin and by which a state of the liquid crystal is altered between a scattering state and a transparent state in response to a voltage applied from the exterior, instead of the TN-type liquid crystal, in the literature, International Symposium Digest of SID, pp. 227-230 (1990) described previously. Still further, in this article, thin film transistors are fabricated by using a polycrystalline silicon thin film.
Further JP-A-2-12291 can also be cited as a literature, in which the projection-type display apparatus is described. In this literature, the light source is located, deviated from the central axis and the projection lens is disposed at a position symmetric to the light source with respect to the central axis.
Among the above prior art techniques, in which the TN-type liquid crystal is driven by thin film transistors, there is a problem that since the thin film transistors themselves and the parts of wiring and electrodes for rows and columns cannot transmit light, the area of one pixel electrode effective for the display is decreased and the effective area is inevitably reduced usually to about 10% to 30% of the total area. Further, because of the light absorption by a polarizing plate inevitable for the TN-type liquid crystal, the light intensity is reduced below 1/2 by the polarizing plate. Therefore, there was a problem that since the transmittance of the liquid crystal panel is extremely small, a light source consuming high electric power is inevitably used in order to increase the brightness on the screen.
Further, even by those using polymer-dispersed-type liquid crystal, although it is possible to eliminate light loss by the polarizing plate, the area occupied by the thin film transistors and the parts of wiring and electrodes is still great, the transmittance of the liquid crystal panel is still insufficient.
Furthermore, in a conventional projection-type display apparatus, since the light source and the projection lens are located symmetrically with respect to the central axis, not only it is not possible to reduce the size satisfactorily, but also an amateur cannot exchange the lamp when the lamp serving as the light source is damaged. In the last case he or she should send the display apparatus to the maker so as to exchange the damaged lamp. This is because the resolution is extremely lowered, if a deviation of the optical axis, which is as small as several micrometers, is caused on the exchange of the light source.